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To configure netcup’s provider firewall, select your server in the Server Control Panel (SCP), open Firewall, create a policy with the rules you need, then assign and save it. The key UDP detail is that the firewall does not track UDP sessions: to allow a UDP request and its reply, define rules for both directions. For netcup’s DNS example, allow UDP ingress to destination port 53 and UDP egress from source port 53.
Before you enable a restrictive policy
Decide which traffic your server must receive and which outbound services it needs. Once you save a custom ingress rule, unmatched ingress traffic is implicitly dropped. Egress works independently: once you define and save a custom egress rule, unmatched egress traffic is implicitly dropped too. A policy can therefore interrupt services even when the rules you added look correct.
If no policy is assigned, netcup’s help page says the firewall automatically allows all traffic. An unassigned firewall is not a protective policy.
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Set up and assign a firewall policy in SCP
- Select the server: In SCP, select the server and open Firewall. If the server has not been configured for the firewall, click Configure and confirm.
- Create a policy: Under Firewall Policies, create a policy and give it a name.
- Add rules: For each rule, choose its direction, protocol, action, source, source ports, destination and destination ports. The documented actions include ACCEPT and DROP; REJECT is not supported. DROP silently discards the traffic without replying.
- Assign the policy: Choose Edit Policies, select your policy and apply it to the server.
- Save and verify: Save the changes. The guide says they apply immediately; a server restart is not required.
Why UDP needs rules in both directions
TCP connections have session state the firewall can track, so it can recognize replies to a server-originated TCP connection. UDP has no established session for the firewall to track. A rule that permits a UDP request does not, by itself, permit the response in the other direction.
For a DNS server, netcup’s example is:
- Incoming queries: UDP ingress to destination port 53.
- Replies to queries: UDP egress from source port 53.
Apply the same principle to other UDP services: identify which side sends the request and which port the service uses for its response, then explicitly permit the required traffic in each direction. Do not open all UDP traffic unless that is genuinely intended.
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Choose rules without blocking necessary traffic
netcup recommends creating ingress rules only. If you add egress restrictions, allow the outbound traffic your server actually needs. Examples in the guide include web connections, NTP, DHCP and client connections; DHCP may only be needed during server installation, and NTP depends on the configured time server.
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For its examples, netcup specifies these rules:
| Purpose | Direction and protocol | Port condition |
|---|---|---|
| Allow client web connections | EGRESS TCP | Destination ports 80 and 443 |
| Allow NTP requests and answers | EGRESS UDP for requests; INGRESS UDP for answers | Destination port 123 for requests; source port 123 for answers |
These are examples, not a complete policy for every server. Check the applications and operating-system services you use before restricting outbound traffic.
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Rule priority, changes and mail traffic
Rules are evaluated in priority order: the first applicable rule takes precedence, and lower-priority rules are ignored for that traffic. Reorder your rules when necessary so the intended rule is reached first.
After a rule change, an already established connection may continue under its existing state and is not necessarily rechecked against the new rules. Do not treat a connection that remains open as proof that a new connection will be allowed by the updated policy.
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netcup’s default mail-block policy blocks SMTP traffic. Remove that policy in SCP only if you intend to run a mail service and have configured it deliberately.
Rule and policy limits
netcup’s help page states a limit of up to 500 active rules per server and public network interface, and up to 500 rules in one policy. A rule can contain up to 100 source or destination IP addresses; each address combination counts separately.
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Check firewall availability for your server
Official netcup pages give different generation thresholds. The Help Center says the firewall is available for server products from Generation 9, with older generations to be equipped gradually and no exact schedule. A netcup announcement dated 9 December 2025 says it is available for Generation 12 onward and describes VPS x86, VPS ARM, root-server and vGPU-server products. The pages do not explain the difference. Check whether Firewall appears for your server in SCP rather than assuming availability from its generation.
Sources: netcup Help Center: Firewall (last updated 23 July 2026); netcup’s 9 December 2025 firewall announcement.
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